The present invention relates to a device for measuring the fluid flow rate in a conduit and, in particular, to a turbine-type flow meter for accurately measuring the air flow rate in the duct work of a heating, ventilating and air conditioning (HVAC) system.
The fluid flow velocity profile in a conduit is not uniform with velocity being greatest at the center of the conduit and theoretically zero at the conduit walls. When using velocity to measure flow rate, consideration must be given to the velocity distribution across the conduit. Therefore, devices providing only single point measurement cannot accurately measure the flow rate through a conduit.
One commonly used single point measurement device is the hot wire anemometer which uses an electrically heated wire placed in the fluid flow stream to measure flow velocity as a function of the rate of heat transfer between the wire and the fluid. Another single point measurement device is the pitot tube which measures the difference between the total and static pressure of the flow which can be used to calculate velocity. The obstruction meter is a third type of single point measurement device. Common forms of obstruction meters are the venturi, the flow nozzle and the orifice. With each of these obstruction meters, the basic meter restricts the path of the flowing fluid causing changes in velocity and concurrent changes in pressure. Velocity can be calculated from pressure measurements made at the location of the restriction and at a point upstream therefrom.
To determine the aveage flow velocity within a conduit using single point measurement technology, an array consisting of many point sensors proportional spaced throughout the flow channel must be used. Due to the large number of sensors often required, these technologies become expensive. Another disadvantage of these technologies is that the sensors generate analog signals which must be converted to a digital format to be used by state-of-the-art digital controllers. An additional drawback of these technologies is that they require a length of at least eight diameters of unobstructed flow upstream from the array to obtain an accurate air flow velocity measurement.
Another flow measurement technique uses ultrasonic waves transmitted across the conduit to measure flow velocity. A transmitter which emits sound waves and a receiver are used to measure the speed of the sound waves which travel at different speeds depending on the fluid flow rate. This technique requires somewhat complex electronic circuitry to produce the sound pulse, to transmit and receive that pulse, and finally to measure the amount of time it takes that pulse to travel across the conduit. Furthermore, since the transducer output is an analog signal, this signal must be converted to a digital signal for use by a digital controller.
A forth flow measuring device is the turbine flow meter. This device uses eight or more blades radiating out from a shaft arranged parallel to the air flow which causes the device to spin (similar to a propeller). A magnetic pick-up or similar means counts the pulses and an air flow measurement is calculated based on the pulses detected by the pick-up. This device is generally no larger than 10 inches in diameter and provides essentially a single point measurement. Typically the turbine flow meter is used only for spot measurements and is removed from the duct after measurements are completed.